HTTP Working Group M. West
Internet-Draft Google, Inc
Updates: 6265 (if approved) M. Goodwin
Intended status: Standards Track Mozilla
Expires: December 22, 2016 June 20, 2016
Same-Site Cookiesdraft-ietf-httpbis-cookie-same-site-00
Abstract
This document updates RFC6265 by defining a "SameSite" attribute
which allows servers to assert that a cookie ought not to be sent
along with cross-site requests. This assertion allows user agents to
mitigate the risk of cross-origin information leakage, and provides
some protection against cross-site request forgery attacks.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ .
Working Group information can be found at http://httpwg.github.io/ ;
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/cookie-same-site .
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 22, 2016.
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Internet-Draft Same-Site Cookies June 20161. IntroductionSection 8.2 of [RFC6265] eloquently notes that cookies are a form of
ambient authority, attached by default to requests the user agent
sends on a user's behalf. Even when an attacker doesn't know the
contents of a user's cookies, she can still execute commands on the
user's behalf (and with the user's authority) by asking the user
agent to send HTTP requests to unwary servers.
Here, we update [RFC6265] with a simple mitigation strategy that
allows servers to declare certain cookies as "same-site", meaning
they should not be attached to "cross-site" requests (as defined in
section 2.1).
Note that the mechanism outlined here is backwards compatible with
the existing cookie syntax. Servers may serve these cookies to all
user agents; those that do not support the "SameSite" attribute will
simply store a cookie which is attached to all relevant requests,
just as they do today.
1.1. Goals
These cookies are intended to provide a solid layer of defense-in-
depth against attacks which require embedding an authenticated
request into an attacker-controlled context:
1. Timing attacks which yield cross-origin information leakage (such
as those detailed in [pixel-perfect]) can be substantially
mitigated by setting the "SameSite" attribute on authentication
cookies. The attacker will only be able to embed unauthenticated
resources, as embedding mechanisms such as "<iframe>" will yield
cross-site requests.
2. Cross-site script inclusion (XSSI) attacks are likewise mitigated
by setting the "SameSite" attribute on authentication cookies.
The attacker will not be able to include authenticated resources
via "<script>" or "<link>", as these embedding mechanisms will
likewise yield cross-site requests.
3. Cross-site request forgery (CSRF) attacks which rely on top-level
navigation (HTML "<form>" POSTs, for instance) can also be
mitigated by treating these navigational requests as "cross-
site".
4. Same-site cookies have some marginal value for policy or
regulatory purposes, as cookies which are not delivered with
cross-site requests cannot be directly used for tracking
purposes. It may be valuable for an origin to assert that its
cookies should not be sent along with cross-site requests in
order to limit its exposure to non-technical risk.
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Internet-Draft Same-Site Cookies June 20161.2. Examples
Same-site cookies are set via the "SameSite" attribute in the "Set-
Cookie" header field. That is, given a server's response to a user
agent which contains the following header field:
Set-Cookie: SID=31d4d96e407aad42; SameSite=Strict
Subsequent requests from that user agent can be expected to contain
the following header field if and only if both the requested resource
and the resource in the top-level browsing context match the cookie.
2. Terminology and notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].
Two sequences of octets are said to case-insensitively match each
other if and only if they are equivalent under the "i;ascii-casemap"
collation defined in [RFC4790].
The terms "active document", "ancestor browsing context", "browsing
context", "document", "WorkerGlobalScope", "sandboxed origin browsing
context flag", "parent browsing context", "the worker's Documents",
"nested browsing context", and "top-level browsing context" are
defined in [HTML].
"Service Workers" are defined in the Service Workers specification
[SERVICE-WORKERS].
The term "origin", the mechanism of deriving an origin from a URI,
and the "the same" matching algorithm for origins are defined in
[RFC6454].
"Safe" HTTP methods include "GET", "HEAD", "OPTIONS", and "TRACE", as
defined in Section 4.2.1 of [RFC7231].
The term "public suffix" is defined in a note in Section 5.3 of
[RFC6265] as "a domain that is controlled by a public registry". For
example, "example.com"'s public suffix is "com". User agents SHOULD
use an up-to-date public suffix list, such as the one maintained by
Mozilla at [PSL].
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An origin's "registrable domain" is the origin's host's public suffix
plus the label to its left. That is, "https://www.example.com"'s
registrable domain is "example.com". This concept is defined more
rigorously in [PSL].
The term "request", as well as a request's "client", "current url",
"method", and "target browsing context", are defined in [FETCH].
2.1. "Same-site" and "cross-site" Requests
A request is "same-site" if its target's URI's origin's registrable
domain is an exact match for the request's initiator's "site for
cookies", and "cross-site" otherwise. To be more precise, for a
given request ("request"), the following algorithm returns "same-
site" or "cross-site":
1. If "request"'s client is "null", return "same-site".
2. Let "site" be "request"'s client's "site for cookies" (as defined
in the following sections).
3. Let "target" be the registrable domain of "request"'s current
url.
4. If "site" is an exact match for "target", return "same-site".
5. Return "cross-site".
2.1.1. Document-based requests
The URI displayed in a user agent's address bar is the only security
context directly exposed to users, and therefore the only signal
users can reasonably rely upon to determine whether or not they trust
a particular website. The registrable domain of that URI's origin
represents the context in which a user most likely believes
themselves to be interacting. We'll label this domain the "top-level
site".
For a document displayed in a top-level browsing context, we can stop
here: the document's "site for cookies" is the top-level site.
For documents which are displayed in nested browsing contexts, we
need to audit the origins of each of a document's ancestor browsing
contexts' active documents in order to account for the "multiple-
nested scenarios" described in Section 4 of [RFC7034]. These
document's "site for cookies" is the top-level site if and only if
the document and each of its ancestor documents' origins have the
same registrable domain as the top-level site. Otherwise its "site
for cookies" is the empty string.
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Given a Document ("document"), the following algorithm returns its
"site for cookies" (either a registrable domain, or the empty
string):
1. Let "top-document" be the active document in "document"'s
browsing context's top-level browsing context.
2. Let "top-origin" be the origin of "top-document"'s URI if "top-
document"'s sandboxed origin browsing context flag is set, and
"top-document"'s origin otherwise.
3. Let "documents" be a list containing "document" and each of
"document"'s ancestor browsing contexts' active documents.
4. For each "item" in "documents":
1. Let "origin" be the origin of "item"'s URI if "item"'s
sandboxed origin browsing context flag is set, and "item"'s
origin otherwise.
2. If "origin"'s host's registrable domain is not an exact match
for "top-origin"'s host's registrable domain, return the
empty string.
5. Return "top-site".
2.1.2. Worker-based requests
Worker-driven requests aren't as clear-cut as document-driven
requests, as there isn't a clear link between a top-level browsing
context and a worker. This is especially true for Service Workers
[SERVICE-WORKERS], which may execute code in the background, without
any document visible at all.
Note: The descriptions below assume that workers must be same-origin
with the documents that instantiate them. If this invariant changes,
we'll need to take the worker's script's URI into account when
determining their status.
2.1.2.1. Dedicated and Shared Workers
Dedicated workers are simple, as each dedicated worker is bound to
one and only one document. Requests generated from a dedicated
worker (via "importScripts", "XMLHttpRequest", "fetch()", etc) define
their "site for cookies" as that document's "site for cookies".
Shared workers may be bound to multiple documents at once. As it is
quite possible for those documents to have distinct "site for cookie"
values, the worker's "site for cookies" will be the empty string in
cases where the values diverge, and the shared value in cases where
the values agree.
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Given a WorkerGlobalScope ("worker"), the following algorithm returns
its "site for cookies" (either a registrable domain, or the empty
string):
1. Let "site" be "worker"'s origin's host's registrable domain.
2. For each "document" in "worker"'s Documents:
1. Let "document-site" be "document"'s "site for cookies" (as
defined in Section 2.1.1).
2. If "document-site" is not an exact match for "site", return
the empty string.
3. Return "site".
2.1.2.2. Service Workers
Service Workers are more complicated, as they act as a completely
separate execution context with only tangential relationship to the
Document which registered them.
Requests which simply pass through a service worker will be handled
as described above: the request's client will be the Document or
Worker which initiated the request, and its "site for cookies" will
be those defined in Section 2.1.1 and Section 2.1.2.1
Requests which are initiated by the Service Worker itself (via a
direct call to "fetch()", for instance), on the other hand, will have
a client which is a ServiceWorkerGlobalScope. Its "site for cookies"
will be the registrable domain of the Service Worker's URI.
Given a ServiceWorkerGlobalScope ("worker"), the following algorithm
returns its "site for cookies" (either a registrable domain, or the
empty string):
1. Return "worker"'s origin's host's registrable domain.
3. Server Requirements
This section describes extensions to [RFC6265] necessary to implement
the server-side requirements of the "SameSite" attribute.
3.1. Grammar
Add "SameSite" to the list of accepted attributes in the "Set-Cookie"
header field's value by replacing the "cookie-av" token definition in
Section 4.1.1 of [RFC6265] with the following ABNF grammar:
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cookie-av = expires-av / max-age-av / domain-av /
path-av / secure-av / httponly-av /
samesite-av / extension-av
samesite-av = "SameSite" / "SameSite=" samesite-value
samesite-value = "Strict" / "Lax"
3.2. Semantics of the "SameSite" Attribute (Non-Normative)
The "SameSite" attribute limits the scope of the cookie such that it
will only be attached to requests if those requests are "same-site",
as defined by the algorithm in Section 2.1. For example, requests
for "https://example.com/sekrit-image" will attach same-site cookies
if and only if initiated from a context whose "site for cookies" is
"example.com".
If the "SameSite" attribute's value is "Strict", or if the value is
invalid, the cookie will only be sent along with "same-site"
requests. If the value is "Lax", the cookie will be sent with "same-
site" requests, and with "cross-site" top-level navigations, as
described in Section 4.1.1.
The changes to the "Cookie" header field suggested in Section 4.3
provide additional detail.
4. User Agent Requirements
This section describes extensions to [RFC6265] necessary in order to
implement the client-side requirements of the "SameSite" attribute.
4.1. The "SameSite" attribute
The following attribute definition should be considered part of the
the "Set-Cookie" algorithm as described in Section 5.2 of [RFC6265]:
If the "attribute-name" case-insensitively matches the string
"SameSite", the user agent MUST process the "cookie-av" as follows:
1. If "cookie-av"'s "attribute-value" is not a case-insensitive
match for "Strict" or "Lax", ignore the "cookie-av".
2. Let "enforcement" be "Lax" if "cookie-av"'s "attribute-value" is
a case-insensitive match for "Lax", and "Strict" otherwise.
3. Append an attribute to the "cookie-attribute-list" with an
"attribute-name" of "SameSite" and an "attribute-value" of
"enforcement".
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Internet-Draft Same-Site Cookies June 20164.1.1. "Strict" and "Lax" enforcement
By default, same-site cookies will not be sent along with top-level
navigations. As discussed in Section 5.2, this might or might not be
compatible with existing session management systems. In the
interests of providing a drop-in mechanism that mitigates the risk of
CSRF attacks, developers may set the "SameSite" attribute in a "Lax"
enforcement mode that carves out an exception which sends same-site
cookies along with cross-site requests if and only if they are top-
level navigations which use a "safe" (in the [RFC7231] sense) HTTP
method.
Lax enforcement provides reasonable defense in depth against CSRF
attacks that rely on unsafe HTTP methods (like "POST"), but do not
offer a robust defense against CSRF as a general category of attack:
1. Attackers can still pop up new windows or trigger top-level
navigations in order to create a "same-site" request (as
described in section 2.1), which is only a speedbump along the
road to exploitation.
2. Features like "<link rel='prerender'>" [prerendering] can be
exploited to create "same-site" requests without the risk of user
detection.
When possible, developers should use a session management mechanism
such as that described in Section 5.2 to mitigate the risk of CSRF
more completely.
4.2. Monkey-patching the Storage Model
Note: There's got to be a better way to specify this. Until I figure
out what that is, monkey-patching!
Alter Section 5.3 of [RFC6265] as follows:
1. Add "samesite-flag" to the list of fields stored for each cookie.
This field's value is one of "None", "Strict", or "Lax".
2. Before step 11 of the current algorithm, add the following:
1. If the "cookie-attribute-list" contains an attribute with an
"attribute-name" of "SameSite", set the cookie's "samesite-
flag" to "attribute-value" ("Strict" or "Lax"). Otherwise,
set the cookie's "samesite-flag" to "None".
2. If the cookie's "samesite-flag" is not "None", and the
request which generated the cookie's client's "site for
cookies" is not an exact match for "request-uri"'s host's
registrable domain, then abort these steps and ignore the
newly created cookie entirely.
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Internet-Draft Same-Site Cookies June 20164.3. Monkey-patching the "Cookie" header
Note: There's got to be a better way to specify this. Until I figure
out what that is, monkey-patching!
Alter Section 5.4 of [RFC6265] as follows:
1. Add the following requirement to the list in step 1:
* If the cookie's "samesite-flag" is not "None", and the HTTP
request is cross-site (as defined in Section 2.1 then exclude
the cookie unless all of the following statements hold:
1. "samesite-flag" is "Lax"
2. The HTTP request's method is "safe".
3. The HTTP request's target browsing context is a top-level
browsing context.
Note that the modifications suggested here concern themselves only
with the "site for cookies" of the request's client, and the
registrable domain of the resource being requested. The cookie's
"domain", "path", and "secure" attributes do not come into play for
these comparisons.
5. Authoring Considerations5.1. Defense in depth
"SameSite" cookies offer a robust defense against CSRF attack when
deployed in strict mode, and when supported by the client. It is,
however, prudent to ensure that this designation is not the extent of
a site's defense against CSRF, as same-site navigations and
submissions can certainly be executed in conjunction with other
attack vectors such as cross-site scripting.
Developers are strongly encouraged to deploy the usual server-side
defenses (CSRF tokens, ensuring that "safe" HTTP methods are
idempotent, etc) to mitigate the risk more fully.
Additionally, client-side techniques such as those described in
[app-isolation] may also prove effective against CSRF, and are
certainly worth exploring in combination with "SameSite" cookies.
5.2. Top-level Navigations
Setting the "SameSite" attribute in "strict" mode provides robust
defense in depth against CSRF attacks, but has the potential to
confuse users unless sites' developers carefully ensure that their
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session management systems deal reasonably well with top-level
navigations.
Consider the scenario in which a user reads their email at MegaCorp
Inc's webmail provider "https://example.com/". They might expect
that clicking on an emailed link to "https://projects.com/secret/project" would show them the secret project that they're authorized
to see, but if "projects.com" has marked their session cookies as
"SameSite", then this cross-site navigation won't send them along
with the request. "projects.com" will render a 404 error to avoid
leaking secret information, and the user will be quite confused.
Developers can avoid this confusion by adopting a session management
system that relies on not one, but two cookies: one conceptualy
granting "read" access, another granting "write" access. The latter
could be marked as "SameSite", and its absence would provide a
reauthentication step before executing any non-idempotent action.
The former could drop the "SameSite" attribute entirely, or choose
the "Lax" version of enforcement, in order to allow users access to
data via top-level navigation.
5.3. Mashups and Widgets
The "SameSite" attribute is inappropriate for some important use-
cases. In particular, note that content intended for embedding in a
cross-site contexts (social networking widgets or commenting
services, for instance) will not have access to such cookies. Cross-
site cookies may be required in order to provide seamless
functionality that relies on a user's state.
Likewise, some forms of Single-Sign-On might require authentication
in a cross-site context; these mechanisms will not function as
intended with same-site cookies.
6. Privacy Considerations6.1. Server-controlled
Same-site cookies in and of themselves don't do anything to address
the general privacy concerns outlined in Section 7.1 of [RFC6265].
The attribute is set by the server, and serves to mitigate the risk
of certain kinds of attacks that the server is worried about. The
user is not involved in this decision. Moreover, a number of side-
channels exist which could allow a server to link distinct requests
even in the absence of cookies. Connection and/or socket pooling,
Token Binding, and Channel ID all offer explicit methods of
identification that servers could take advantage of.
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